The present invention relates to cancellation of substrate noise in a semiconductor device, and more particularly, to a passive cancellation method of substrate noise for a buck converter.
A buck converter is a well-known device in the art. Switching of the buck converter injects noise into the substrate, which affects analog circuit blocks. Traditional isolation methods do not reduce substrate noise enough.
U.S. Pat. No. 6,040,728 of Dale H. Nelson, et. al., provides an active substrate noise injection cancellation method that requires a specially designed operational amplifier, which is complicated.
The paper entitled “Mechanism of Common-Mode Noise Reduction in Balanced Boost Switching Converter” (35th Annual IEEE Power Electronics Specialists Conference, Pages: 1115-1120 Vol. 2, 20-25, Jun. 2004) by M. Shoyama et. al., proposes a mechanism of common-mode noise reduction that utilizes a big load inductor in a boost converter to reduce noise on frame ground.
FIG. 1 schematically shows a circuit structure of a buck converter. In FIG. 1, for a buck converter without an external Schottky diode, due to parasitic inductance of the power supply, points A and B are noisy and have opposite phases. Substrate noise is injected through one n-well/p-substrate junction from point A and through two junctions from point B, leading to different noise magnitudes.
It would be advantageous to have a method that can effectively reduce substrate noise in a buck converter generated by switching.